1. Field of the Invention
The present invention relates to a developer for developing an electrostatic latent image, and more particularly to a negatively-chargeable non-magnetic one-component developer. The present invention also relates to an image forming method using the developer.
2. Discussion of the Background
Electrophotographic image forming methods typically include the following processes:
(1) charging a surface of a photoreceptor to such that the surface has a predetermined potential (charging process);
(2) irradiating the charged surface of the photoreceptor with light to form an electrostatic latent image on the surface of the photoreceptor (light irradiating process);
(3) developing the electrostatic latent image with a developer including a colored toner to form a toner image on the surface of the photoreceptor (developing device);
(4) transferring the toner image onto a receiving material optionally via an intermediate transfer medium (transfer process); and
(5) fixing the toner image on the receiving material by applying heat, etc. thereto, resulting in formation of a copy (fixing process).
Dry developing methods used for electrophotography and electrostatic recording methods are broadly classified into two-component developing methods using a two-component developer including a toner and a carrier and one component developing methods using a one-component developer including a toner and no carrier.
The two-component developing methods have an advantage in that toner images having good image qualities can be relatively stably produced but have a drawback in that the carrier deteriorates and in addition the ratio of the toner and the carrier in the developer varies and thereby image qualities tend to vary when the developer is used for a long period of time. In addition, it is troublesome to keep the developing devices maintained and it is hard to miniaturize the developing devices. Therefore, the one-component developing methods using a one-component developer (hereinafter referred to as a one-component toner) attract attention now.
In the one-component developing methods, a developer (i.e., toner) is typically fed by at least one toner feeding member and the fed toner develops an electrostatic latent image formed on an image bearing member, resulting in formation of a visual (toner) image. In addition, a toner thickness controlling member is arranged so as to face the toner feeding member to form a toner layer having an even thickness on the toner feeding member and to frictionally charge the toner particles in the toner layer. Blades are typically used as the toner thickness controlling member. Specifically, toner particles located on and fed by the toner feeding member are pressed by a blade serving as the toner thickness controlling member, and thereby a toner layer having a predetermined thickness can be formed on the toner feeding member while being frictionally charged by the blade. Rollers can be used as the toner thickness controlling member instead of blades.
The charging methods are broadly classified into non-contact charging methods such as wire charging methods which utilize corona discharging, and contact charging methods utilizing charge injection or short-range charging methods utilizing small gap discharging. Recently, contact charging methods are typically used to miniaturize the charging device and reduce the amount of generated oxidizing materials (such as ozone and NOx).
In the developing process, the bottom portion of the toner layer on the toner feeding member (such as developing sleeves), i.e., the bottom portion of the toner layer near the surface of the sleeve, has a large amount of charge, and thereby the bottom portion is strongly attracted by the surface of the sleeve due to reflection power. Therefore, the bottom portion is hardly moved on the surface of the sleeve, and the toner particles in the bottom portion of the toner layer on the sleeve is hardly transferred to an electrostatic latent image. This phenomenon is called “a charge-up phenomenon”. This charge-up phenomenon easily occurs under low humidity conditions.
When the charge-up phenomenon occurs, the upper portion of the toner layer on the sleeve has a small amount of charge, and thereby problems such as a background development problem in that the background area of an image is soiled with toner particles, a toner leaking problem in that toner particles leak from the developing device, and a toner scattering problem in that toner particles are scattered around the developing device are caused. In order to prevent occurrence of such problems, it is preferable to uniformly charge the toner particles in the toner layer. In attempting to prevent occurrence of such problems, the following proposals for improving electric properties of toner have been made.
Specifically, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 09-34159 discloses a toner having a specific dielectric loss tangent and includes a carbon black having a specific average particle diameter in a predetermined amount. One-component toners are required to have a charge property so as to be rapidly charged to have and maintain a predetermined amount of charge. Therefore, the capacity, and resistance of a toner have to be also controlled so as to fall in proper ranges as well as dielectric loss tangent. In other words, good charge properties cannot be imparted to a one-component developer (i.e., toner) only by controlling the dielectric loss tangent of the toner.
JP-A 09-34174 discloses a toner including an acidic carbon black having a pH of not higher than 7. Since acidic carbon blacks have poor dispersibility in resins, the resultant toner tends to have a high dielectric loss tangent, and therefore the toner cannot stably maintain a charge.
JP-A 2004-333970 discloses a toner having a high dielectric loss tangent. Therefore, the toner has poor charging ability, and thereby a problem in that the toner has a small amount of charge occurs. In addition, the materials constituting the toner are poorly dispersed in the toner.
JP-A 09-204063 discloses a toner in which a wax is dispersed well. However, the capacitance, resistivity and dielectric loss tangent of the toner, by which the charging ability of the toner can be represented, are not specified. Therefore, a one-component developer (i.e., toner) having good charging ability cannot be provided. Since the dispersion state of the constitutional materials in the toner and the charging ability of the toner change depending on the dielectric loss tangent of the toner, it is important to specify the absolute value of the dielectric loss tangent of the toner.
JP-A 11-295923 discloses a toner having a specific dielectric loss tangent at 100 Hz. As a result of the present inventors' study, such a toner cannot satisfy a requirement for one component developers such that the charge of the toner has to be rapidly increased to a desired charge quantity and the charge quantity is maintained. In addition, the dielectric loss tangent described in the specification of JP-A 11-295923 is too small to be balanced with the resistivity and capacitance of the toner, and thereby the toner is defectively charged.
JP-A 2000-3066 discloses a toner having a relatively high dielectric constant. Thereby, the toner has a relatively low dielectric loss tangent. In this case, the toner has a large amount of charge and a low charge rising property.
Because of these reasons, a need exists for a one component toner which can be efficiently charged frictionally at a nip between a developer feeding member and a toner layer thickness controlling blade and which can produce high quality images.